Double emulsified acids and methods for producing and using the same

ABSTRACT

Embodiments of the present disclosure are directed to double emulsified acids, specifically water in oil in water (W/O/W) double emulsions with an acidic interior water phase. The double emulsified acid may include two or more emulsifying agents. In addition to improving the stability of the emulsions, the emulsifying agents may help slow the acid reaction rate. The first emulsifying agent may have an HLB greater than 7 and the second emulsifying agent may have an HLB less than or equal to 7. Other embodiments are directed to methods for producing a double emulsified acid and methods for acidizing a carbonate formation including a double emulsified acid.

BACKGROUND Technical Field

This disclosure relates to double emulsions. More specifically, thisdisclosure relates to acidic double emulsions and methods of preparingand using acidic double emulsions.

Background

In oil and gas drilling, wellbore stimulation is a common treatmentperformed in subterranean formations to enhance or restore theproductivity of oil and gas from a wellbore. Acid treatments, asdetailed in this disclosure, may be used for wellbore stimulation.Acidizing is a stimulation acid treatment technique in which a treatmentfluid comprising aqueous acid solution is delivered into thesubterranean formation to dissolve acid-soluble materials, such ascarbonates. In a functioning wellbore, the formation rock is porous andallows for the flow of gas and oil from the formation to the wellbore.Carbonate formations can block or obstruct this flow of gas or oil.These carbonate formations may impact the productivity of a wellbore.Acidic solutions may be utilized to acidize the carbonate formations andimprove the productivity of the wellbore. This can increase thepermeability of a treatment zone and enhance well production byincreasing the effective wellbore radius.

Conventional acids react very quickly in carbonate formations. Thereaction is so rapid in great temperatures that it is impossible foracid to penetrate, or wormhole, more than a few inches into theformation. In such cases, the acid is rendered ineffective instimulating the wellbore. Compositions known in the art that attempt tosolve this problem, such as single emulsified acids, are relativelyviscous and cannot be efficiently pumped at fast rates or intoenvironments with extreme pressures, that is, static reservoir pressuresabove 10,000 pounds per square inch.

SUMMARY

Accordingly, ongoing needs exist for acid formulations which slow theacid reaction rate to allow for deeper wellbore penetration by acidizingstimulation. Further, needs exist for acid formulations with a retardedreaction rate and rheological properties conducive to fast pumping ratesand high temperature and high pressure (“HTHP”) environments. As used inthis disclosure, HTHP environments refer to environments wheretemperatures can range from 150° C. to 320° C. and static reservoirpressures can range from 10,000 pounds per square inch (psi) to 20,000psi.

Embodiments of the present disclosure are directed to double emulsifiedacids, specifically water in oil in water (W/O/W) double emulsions withan acidic interior water phase. The double emulsified acid may includetwo or more emulsifying agents. In addition to improving the stabilityof the emulsions, the emulsifying agents may help slow the acid reactionrate.

In one embodiment, a double emulsified acid comprises a continuousaqueous phase and a non-continuous invert emulsion phase. Thenon-continuous invert emulsion phase may comprise an exterior oil phase,an internal acid phase, and a first emulsifying agent with ahydrophilic-lipophilic balance (HLB) greater than 7. A secondemulsifying agent having an HLB less than or equal to 7 may stabilizethe non-continuous invert emulsion phase within the continuous aqueousphase. The internal acid phase may comprise from 1 percent by weight(wt. %) to 99 wt. % of at least one acid and from 1 wt. % to 99 wt. %water, based on the total weight of the internal acid phase.

In another embodiment, a method for producing a double emulsified acidcomprises introducing an aqueous acid to a first vessel and introducingan oil-based composition and a first emulsifying agent having an HLBgreater than 7 to a second vessel. The method further comprises mixingthe contents of the first vessel and the contents of the second vesselat a first shear rate to form an invert emulsion. The method may furthercomprise mixing the invert emulsion with water and a second emulsifyingagent having an HLB less than or equal to 7 at a second shear rate toform a double emulsified acid.

In yet another embodiment, a method for acidizing a carbonate formationcomprises introducing a double emulsified acid to the carbonateformation. In such embodiments, the double emulsified acid may comprisea continuous aqueous phase and a non-continuous invert emulsion phase.The non-continuous invert emulsion phase may comprise an exterior oilphase, an internal acid phase, and a first emulsifying agent having anHLB greater than 7. In one or more embodiments, a second emulsifyingagent having an HLB less than or equal to 7 stabilizes thenon-continuous invert emulsion phase within the continuous aqueousphase. In other embodiments, the internal acid phase comprises from 1percent by weight (wt. %) to 99 wt. % of at least one acid and from 1wt. % to 99 wt. % water, based on the total weight of the internal acidphase. The method for acidizing the carbonate formation may furthercomprise maintaining the double emulsified acid in the carbonateformation such that the double emulsion breaks, where the at least oneacid dissolves at least part of the carbonate formation, acidizing thecarbonate formation.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 depicts a microscopic image of a double emulsified acid,according to one or more embodiments of the present disclosure.

The embodiments set forth in the drawings are illustrative in nature andnot intended to be limiting to the claims. Moreover, individual featuresof the drawings will be more fully apparent and understood in view ofthe detailed description.

DETAILED DESCRIPTION

Embodiments of the present disclosure are directed to a doubleemulsified acid comprising a continuous aqueous phase, a non-continuousinvert emulsion phase, and an emulsifying agent with an HLB less than orequal to 7. In one or more embodiments, the non-continuous invertemulsion phase comprises an exterior oil phase, an internal acid phase,and an emulsifying agent with an HLB greater than 7. In otherembodiments, the internal acid phase comprises from 1 wt. % to 99 wt. %of at least one acid and from 1 wt. % to 99 wt. % water based on thetotal weight of the internal acid phase.

As used in the present disclosure, an “emulsion” refers to a mixturecomprising immiscible phases, where one immiscible phase is dispersedevenly throughout the other immiscible phase. The phase that isdispersed is referred to as the non-continuous phase or internal phase.The other phase, the phase that surrounds the dispersed phase, isreferred to as the continuous phase or exterior phase. Most emulsionsdescribed in the present disclosure involve immiscible aqueous phasesand oil phases. Emulsions with external water phases are often referredto as just “emulsions” while emulsions with external oil phases may bereferred to as “invert emulsions.”

As used in the present disclosure, a “double emulsion” refers to eitheran emulsion or an invert emulsion where the non-continuous phase,dispersed evenly throughout the continuous phase, comprises either anemulsion or an invert emulsion. By way of example and not limitation,one such double emulsion would include an oil based continuous phasewith an emulsion non-continuous phase, where the non-continuous phasecomprises an emulsion with both an aqueous exterior phase and an oilbased internal phase.

As used in the present disclosure, “HLB” refers to Griffin'shydrophilic-lipophilic balance of a molecule. The HLB value of amolecule is a measure of the degree to which it is hydrophilic orlipophilic. HLB may be calculated according to Equation 1:

$\begin{matrix}{{HLB} = {20*\frac{M_{h}}{M}}} & {{Eq}.\mspace{14mu}(1)}\end{matrix}$where M_(b) is the molecular mass of the hydrophilic portion of themolecule and M is the molecular mass of the whole molecule. Griffin'sHLB values range from 0 to 20 in which a value of 0 indicates anabsolutely hydrophobic/lipophilic molecule and a value of 20 correspondsto an absolutely hydrophilic/lipophobic molecule. Generally, moleculeshaving an HLB less than 10 are lipid soluble, molecules having an HLBgreater than 10 are water soluble, and molecules with an HLB between 3and 16 have some surfactant properties.

As further background, a subterranean wellbore, or just “wellbore,” is adrilled or bored hole that includes the open hole and the rock, dirt,sand, or stone that bounds the open hole. A wellbore is formed byinserting a drill string into a previously drilled hole. The drillstring may then be rotated about an annular axis causing the drill bitto cut into the surrounding subterranean formation and thereby expandingthe hole. The surrounding formation may vary in composition and mayinclude rock, dirt, sand, stone, or combinations thereof.

The wellbore is generally drilled in proximity to a target hydrocarbonformation. The formation surrounding the wellbore is porous and allowsfor the target oil or gas to flow from the formation to the wellbore.This flowing of target oil or gas to the wellbore enables the extractionof the target oil or gas through the wellbore.

During drilling operations, geologic carbonate formations may be presentnear or around the desired oil or gas bearing formations and wellbore.These carbonate formations can negatively impact the productivity of awellbore. To mitigate the impact these carbonate formations have on theproductivity of the wellbore, the wellbores may be treated with acidicformulations.

Over time, the production of a wellbore may decrease. This may be due toseveral factors including, but not limited to, obstructions in thewellbore, decreased permeability of the surrounding formation, or damageto the wellbore from drilling or extraction procedures. There are manypotential obstructions that may present in the wellbore. For example,when drilling muds filter into the surrounding formation they canobstruct the pores or channels through which target oil or gas fluidsflow. Alternatively, water injection may also lead to obstructions inthe wellbore. The injected water or other aqueous solutions may haveparticles of size sufficient to obstruct the channels in the formation.

These obstructions may decrease wellbore productivity, a general measureof how much target oil or gas can be extracted from a wellbore. Wellboreproductivity may be measured by the pressure at which the targetformation flows through the wellbore. Alternatively, wellboreproductivity may be quantified by the total mass of target fluidextracted through the wellbore.

Drilling and extraction procedures may affect the surrounding formation.These procedures can cause shifts in the constituent materials of theformation and lead to decreased wellbore productivity. For example,drilling into the formation can cause sandstone or carbonate formationsto shift and obstruct channels that carry target fluids to the wellbore.Alternatively, carbonates and other solids may precipitate out ofdrilling muds, workover fluids, or other solutions used in the drillingand extraction process. These solids can obstruct channels thatfacilitate the flow of target fluids and decrease wellbore productivity.

The purpose of any acid treatment is to improve wellbore productivity.There are three general categories of acid treatments: acid washing,acid fracturing, and acidizing. The type of treatment used generallydepends on the formation composition and formation permeability. Theformation composition may comprise carbonate, sand, shale, sandstone,other geologic formations, or combinations thereof. The formationpermeability is the ability of a fluid to flow through the formation inits natural state.

Acidizing involves adding acid into a wellbore at a pressure less thanfracturing pressure. Because acidizing occurs at a pressure less thanthe corresponding acid fracturing pressure, it is generally easier tocontrol and direct the flow of the acid in an efficient manner. Theproblem with this type of acid treatment, using conventional acids, isthat the acid reacts with the carbonate too quickly and cannot permeate,or wormhole, into less permeable formations. The double emulsified acidsof the present disclosure solve this problem by sequestering the acidicphase within an oil phase and within an exterior aqueous phase.

During acidizing operations, the acid is generally chemically consumedand neutralized as the target formation is dissolved. For a generalacid, HA (with conjugate anion, A−), reacting with a general carbonate,MCO₃, this reaction proceeds according to Equation 2:HA_((aq))+MCO_(3(s))→M_((aq)) ⁺+A_((aq)) ⁻+H₂O_((l))+CO_(2(g))  Eq. (2)As is apparent from the chemical equation, the carbonate is dissolvedinto the aqueous acid solution and carbon dioxide is evolved. Further,the amount of carbon dioxide evolved is directly proportional to theamount of carbonate dissolved. This reaction generally occursinstantaneously and proceeds to completion quickly. Using doubleemulsified acids, this reaction occurs at a slower rate, enabling theacidizing solution to wormhole further into the carbonate formation.

Two substances cannot undergo a chemical reaction unless the constituentmolecules, ions, or atoms come into contact. As a consequence, the morereactant particles that collide with each other per unit time, thefaster the reaction will proceed. Therefore, the rate at which achemical reaction proceeds may be affected by reactant concentrations,temperature, physical states and surface areas of reactants, andproperties of non-reacting compounds in the reaction solution. All ofthese factors can affect the collision rate of reactants.

The double emulsified acids of the present disclosure sequester theacidic reactant particles from the formation wall. Therefore, the acidcan only react with carbonate formations when the double emulsion isbroken, exposing the acid to the formation. As used in the presentdisclosure, the “breaking” of an emulsion or double emulsion refers tothe dissolution of the component phases. When an emulsion breaks, itsinterfacial film is interrupted and droplets of the immiscible phasescoalesce. The requirement of breaking the double emulsion to contact theacid to the formation wall slows down the collision rate of thereactants, namely the acid ions and carbonate solids, thereby slowingdown the reaction rate.

In one or more embodiments, a double emulsified acid comprises acontinuous aqueous phase and a non-continuous invert emulsion phase. Inone or more embodiments, the non-continuous invert emulsion phasecomprises an exterior oil phase, an internal acid phase, and a firstemulsifying agent with an HLB greater than 7. In one or moreembodiments, a second emulsifying agent having an HLB less than or equalto 7 stabilizes the non-continuous invert emulsion phase within thecontinuous aqueous phase. In other embodiments, the internal acid phasecomprises from 1 percent by weight (wt. %) to 99 wt. % of at least oneacid and from 1 wt. % to 99 wt. % water, based on the total weight ofthe internal acid phase.

In one or more embodiments, the continuous aqueous phase comprises from0.1 wt. % to 99.9 wt. % water, based on the total weight of thecontinuous aqueous phase. In other embodiments, the continuous aqueousphase may comprise salts, clay stabilizers, corrosion inhibitors, ororganic solvents. Suitable salts include, by way of example and notlimitation, lithium salts, sodium salts, potassium salts, berylliumsalts, magnesium salts, calcium salts, transition metal salts, halides,oxides, sulfides, selenides, phosphides, sulfates, sulfites, phosphates,and phosphites. In one or more embodiments, the continuous aqueous phasecomprises greater than 0.1 wt. % total dissolved solids, based on thetotal weight of the continuous aqueous emulsion phase. In otherembodiments, the continuous aqueous emulsion phase comprises from 0.1wt. % to 50 wt. %, from 0.1 wt. % to 10 wt. %, from 0.1 wt. % to 5 wt.%, or from 1 wt. % to 10 wt. % based on the total weight of thecontinuous aqueous phase.

In one or more embodiments, the continuous aqueous phase may compriseclay stabilizers. As used in the present disclosure, a clay stabilizerrefers to a chemical additive that prevents the swelling or migration ofclay particles into a wellbore fluid from the surrounding formation.When aqueous fluids are circulated in a wellbore, the water in the fluidcan affect the electrical charge of clay platelets in the surroundingformation. As the electrical charges of the clay platelets are changed,the clay may swell, that is absorb water into the crystalline structureof the clay. As the clay platelets swell, they may more easily break offfrom the surrounding formation and infiltrate the wellbore fluid.Example clay stabilizers include, but are not limited to, ionic salts,polyacrylamide polymers, and polyacrylate polymers.

In one or more embodiments, the clay stabilizer of the continuousaqueous phase may comprise KCl, NaCl, NH₄Cl, tetramethylammoniumchloride (TMAC), or combinations thereof. In other embodiments, thecontinuous aqueous phase comprises from 1 wt. % to 8 wt. % of at leastone clay stabilizer based on the total weight of the continuous aqueousphase. Without being limited by theory, it is believed these claystabilizers may prevent the circulating aqueous fluid from affecting theelectrical properties of the clay platelets, preventing infiltration ofthe clay into the fluid.

In one or more embodiments, the continuous aqueous phase may compriseone or more corrosion inhibitors. Corrosion inhibitors are chemicalsthat may be added to acidic wellbore fluids to prevent the acid in thefluid from corroding iron, steel, and other metal components of thewellbore. In one or more embodiments, a double emulsified acid may bepumped through metal coiled tubing. In such embodiments, corrosioninhibitors in the double emulsified acid protect the integrity of themetal coiled tubing. Additionally, hydrogen sulfide in the wellbore orsurrounding formation may cause corrosion, known in the art as “sourcorrosion.” Suitable corrosion inhibitors may have no effect on thereaction rate between acid and carbonates while reducing the reactionbetween acids (or hydrogen sulfide) and wellbore metals. Examplecorrosion inhibitors include, but are not limited to, amines, ammonia,quaternary ammonium salts, morpholine, oxygen scavengers, and thiourea.

In one or more embodiments, the continuous aqueous phase may compriseone or more organic solvents. Relatively small quantities of organicsolvents may be incorporated into the continuous aqueous phase to allowfor the consistent, uniform dispersion of fluid additives and theinternal invert emulsion phase. In one or more embodiments, thecontinuous aqueous phase comprises from 0.01 wt. % to 5 wt. % organicsolvents, based on the total weight of the continuous aqueous phase. Inother embodiments, the continuous aqueous phase comprises from 0.01 wt.% to 1 wt. %, from 0.1 wt. % to 5 wt. %, from 0.01 wt. % to 0.5 wt. %,or from 0.01 wt. % to 0.1 wt. %, based on the total weight of thecontinuous aqueous phase. One or more organic solvents may include, butare not limited to, acetone, acetonitrile, diesel, dimethylformamide,dimethyl sulfoxide, ethylene glycol, ethyl acetate, hexanes,nitromethane, pentanes, propylene carbonate, tetrahydrofuran, andxylene.

In one or more embodiment double emulsified acids, the non-continuousinvert emulsion phase comprises an exterior oil phase, an internal acidphase, and a first emulsifying agent with an HLB greater than 7. Theexterior oil phase of the non-continuous invert emulsion phase maycomprise mineral oil, synthetic oils derived from plant or animalproducts, diesel, C₁₂-C₂₀ hydrocarbons, or combinations thereof. On suchsynthetic oil, by way of example and not limitation, is safra oil, acommercially available oil from the Safra Company Limited (SaudiArabia). As used in the present disclosure, “C₁₂-C₂₀ hydrocarbons”refers to hydrocarbons, that are compounds comprising hydrogen andcarbon, having from 12 to 20 carbon atoms. In one or more embodiments,the C₁₂-C₂₀ hydrocarbons comprise olefins. In other embodiments, theC₁₂-C₂₀ hydrocarbons comprise linear alpha olefins.

The internal acid phase of the non-continuous invert emulsion phase maycomprise from 1 wt. % to 99 wt. % of at least one acid and from 1 wt. %to 99 wt. % water, based on the total weight of the internal acid phase.In other embodiments, the internal acid phase may comprise from 5 wt. %to 99 wt. %, from 10 wt. % to 99 wt. %, from 25 wt. % to 99 wt. %, from40 wt. % to 99 wt. %, from 25 wt. % to 75 wt. %, or even from 50.1 wt. %to 99 wt. % of at least one acid, based on the total weight of theinternal acid phase. In one or more embodiments, the internal acid phasemay comprise from 1 wt. % to 5 wt. %, from 1 wt. % to 90 wt. %, from 1wt. % to 75 wt. %, from 1 wt. % to 60 wt. %, from 25 wt. % to 75 wt. %,or from 1 wt. % to 49.9 wt. % water, based on the total weight of theinternal acid phase.

Acid strength is characterized by how well a particular acid dissociatesand donates protons. A stronger acid is an acid that dissociates at afrequent rate and donates a comparatively greater concentration ofprotons. A weaker acid does not donate protons as well because itdissociates at a less frequent rate, resulting in a comparatively lesserconcentration of protons. Acid strength can be quantified by thelogarithmic acid dissociation constant, pK_(a). The pK_(a) of an acidmay be defined as the negative logarithm quotient of the concentrationof conjugate anions multiplied by concentration of donated protons,divided by the concentration of undissociated acid. Acids with pK_(a)sless than 5 may be desirable for some acidizing operations because theydissociate at a frequent rate, resulting in a more complete reaction andless unreacted acid remaining in the formation.

In one or more embodiments, the internal acid phase of thenon-continuous invert emulsion phase comprises an acid with a pK_(a)less than or equal to 5. In other embodiments, at least one acid maycomprise an inorganic acid. In still other embodiments, the at least oneacid may comprise an organic acid. The at least one acid of the internalacid phase of the non-continuous invert emulsion phase may compriseacetic acid, chloric acid, formic acid, hydrochloric acid, hydrobromicacid, hydrofluoric acid, hydroiodic acid, nitric acid, oxalic acid,perchloric acid, sulfuric acid, sulfurous acid, or combinations thereof.In other embodiments, the internal acid phase may comprise varioussalts, clay stabilizers, corrosion inhibitors, or organic solvents asdescribed in the present disclosure.

In one or more embodiments, the non-continuous invert emulsion phasecomprises a first emulsifying agent with an HLB greater than 7. In otherembodiments, the first emulsifying agent has an HLB greater than orequal to 10, or even greater than or equal to 12. In one or moreembodiments, a double emulsified acid comprises a second emulsifyingagent with an HLB less than or equal to 7. In other embodiments, thesecond emulsifying agent has an HLB less than or equal to 6, or evenless than or equal to 5.

As used in the present disclosure, an emulsifying agent refers to acompound that lowers the surface tension between two immisciblesubstances. An emulsifying agent, also referred to as an emulsifier, maycomprise surfactants, detergents, wetting agents, dispersants,nanoparticles, polyacrylamides, or combinations thereof. In one or moreembodiments, an emulsifying agent comprises at least one amphiphilicorganic compound. Emulsifying agents can also sequester the acids evenafter the emulsion breaks, further retarding the reaction rate betweenthe acid and carbonates in the formation. In other embodiments, anemulsifying agent comprises ethoxylated alcohols, alkylphenols, orcombinations thereof. In one or more embodiments of the doubleemulsified acids, the first emulsifying agent and the second emulsifyingagent may comprise an ethoxylated alcohol.

In one or more embodiments, a double emulsified acid comprises from 1wt. % to 96 wt. % continuous aqueous phase, based on the total weight ofthe double emulsified acid. In other embodiments, the double emulsifiedacid comprises from 1 wt. % to 45 wt. %, from 5 wt. % to 35 wt. % from10 wt. % to 30 wt. %, from 10 wt. % to 25 wt. %, from 15 wt. % to 30 wt.%, or from 15 wt. % to 25 wt. % continuous aqueous phase, based on thetotal weight of the double emulsified acid.

In one or more embodiments, a double emulsified acid comprises from 1wt. % to 96 wt. % exterior oil phase, based on the total weight of thedouble emulsified acid. In other embodiments, the double emulsified acidcomprises from 1 wt. % to 30 wt. %, from 5 wt. % to 25 wt. %, from 5 wt.% to 20 wt. %, from 10 wt. % to 25 wt. %, or from 10 wt. % to 20 wt. %exterior oil phase, based on the total weight of the double emulsifiedacid.

In one or more embodiments, a double emulsified acid comprises from 1wt. % to 96 wt. % internal acid phase, based on the total weight of thedouble emulsified acid. In other embodiments, the double emulsified acidcomprises from 5 wt. % to 65 wt. %, from 15 wt. % to 65 wt. %, from 30wt. % to 65 wt. %, from 5 wt. % to 45 wt. %, from 15 wt. % to 45 wt. %,or from 30 wt. % to 45 wt. % internal acid phase, based on the totalweight of the double emulsified acid.

In one or more embodiments, a double emulsified acid comprises from 1wt. % to 30 wt. % of a first emulsifying agent with an HLB greater than7, based on the total weight of the double emulsified acid. In otherembodiments, the double emulsified acid comprises from 5 wt. % to 25 wt.%, from 5 wt. % to 20 wt. %, from 10 wt. % to 25 wt. %, or from 10 wt. %to 20 wt. % of a first emulsifying agent with an HLB greater than 7,based on the total weight of the double emulsified acid.

In one or more embodiments, a double emulsified acid comprises from 1wt. % to 30 wt. % of a second emulsifying agent with an HLB less than orequal to 7, based on the total weight of the double emulsified acid. Inother embodiments, the double emulsified acid comprises from 5 wt. % to25 wt. %, from 5 wt. % to 20 wt. %, from 10 wt. % to 25 wt. %, or from10 wt. % to 20 wt. % of a first emulsifying agent with an HLB less thanor equal to 7, based on the total weight of the double emulsified acid.

One or more embodiments of the present disclosure provide a method forproducing a double emulsified acid, the method comprises adding anaqueous acid to a first vessel, adding an oil-based composition and afirst emulsifying agent having an HLB greater than 7 to a second vessel,mixing the contents of the first vessel and the contents of the secondvessel at a first shear rate to form an invert emulsion, and mixing theinvert emulsion with water and a second emulsifying agent with an HLBless than or equal to 7 at a second shear rate to form a doubleemulsified acid. In one or more embodiments, the first shear rate may bedifferent than the second shear rate.

In one or more embodiments, a method for producing a double emulsifiedacid may further comprise adding a corrosion inhibitor, an organicsolvent, a viscosity modifier, or combinations thereof to the firstvessel. In other embodiments, a corrosion inhibitor, an organic solvent,a viscosity modifier, or combinations thereof may be added to the doubleemulsified acid after mixing at a second shear rate.

As used in the present disclosure, an “aqueous acid” refers to an acidicsolution that comprises greater than 0 wt. % of at least one acid andless than 100 wt. % water, based on the total weight of the aqueousacid. The at least one acid in the aqueous acid may refer to organicacids, inorganic acids, or combinations thereof. In other embodiments,the at least one acid in the aqueous acid may refer to acids having apK_(a) less than 5, such as, by way of non-limiting example: aceticacid, chloric acid, formic acid, hydrochloric acid, hydrobromic acid,hydrofluoric acid, hydroiodic acid, nitric acid, oxalic acid, perchloricacid, sulfuric acid, sulfurous acid, or combinations thereof.

In one or more embodiment methods for producing a double emulsifiedacid, the first shear rate is greater than 400 cycles per second (s⁻¹).In other embodiments, the first shear rate is from 400 s⁻¹ to 2000 s⁻¹,from 400 s⁻¹ to 1600 s⁻¹, from 1000 s⁻¹ to 2000 s⁻¹, or from 1000 s⁻¹ to1600 s⁻¹.

In one or more embodiment methods for producing a double emulsifiedacid, the second shear rate less than or equal to 2000 s⁻¹. In otherembodiments, the second shear rate is from 400 s⁻¹ to 2000 s⁻¹, from 400s⁻¹ to 1600 s⁻¹, from 1000 s⁻¹ to 2000 s⁻¹, or from 1000 s⁻¹ to 1600s⁻¹.

In one or more embodiments, a method for acidizing a carbonate formationcomprises introducing a double emulsified acid to the carbonateformation, maintaining the double emulsified acid in the carbonateformation such that the double emulsion breaks, where the at least oneacid of the double emulsified acid dissolves at least part of thecarbonate formation, acidizing the carbonate formation.

In one or more embodiments, the acidizing solution may be need to keptat temperature and pressure conditions different than those near thecarbonate formation. In such embodiments, the double emulsified acid maybe passed through an insulator prior to being introduced to thecarbonate formation. As used in the present context, an insulator refersto a tubular that separates a fluid on the inside of the tubular fromthe environment outside of the tubular. In one or more embodiments, thedouble emulsified acid is pumped through and insulator that runs fromthe surface to the carbonate formation. The insulator may be made out ofmaterials inert to reaction with the double emulsified acid. In one ormore embodiments, the insulator is made out of metal, plastic, or othercomposite materials. Pressure or a vacuum may be applied to the interiorof the insulator to adjust the pressure conditions in the insulator asneeded. Additionally, the insulator may be heated or cooled to adjustthe temperature of the interior of the insulator as needed. As describedpreviously, the formation permeability may affect the rate and pressureat which the double emulsified acid is pumped.

In addition to formation permeability, other factors may contribute tothe requisite pumping pressure and rate required to acidize a carbonateformation. For example, HTHP conditions in the wellbore may affect thepumping rate or pressure required to introduce the acidizing solution toa subterranean carbonate formation. In HTHP environments temperaturescan range from 150° C. to 320° C. and static reservoir pressures canrange from 10,000 pounds per square inch (psi) to 20,000 psi.

Conventional acid emulsions perform poorly in these environments becausethey are too viscous to efficiently pump into the wellbore. The doubleemulsified acids of the present disclosure have less viscosity and canbe efficiently pumped into the wellbore, even in HTHP environments.

EXAMPLES

In the following examples, the produced double emulsified acid iscompared to a conventional emulsified acid. The doubled emulsified acidwas measured as having less viscosity and a slower carbonate reactionrate compared to the conventional emulsified acid formulation.

Comparative Example A

A conventional emulsified acid, Comparative Example A, was prepared byadding 90 milliliters (mL) of diesel and 3 mL of AF-70, a commerciallyavailable emulsifying agent from the Halliburton Company with an HLBgreater than 7, to a multimixer. Then, the diesel and emulsifying agentwere mixed at a shear rate of from 400 to 1000 revolutions per minute(rpm) for 1 minute. Finally, 210 mL of a 20 wt. % hydrochloric acidsolution were added dropwise over 45 minutes while the diesel,emulsifying agent, and acid mixture was mixed at a shear rate of from1000 to 2000 rpm.

Example 1

A double emulsified acid, Example 1, was prepared by first adding 90 mLof diesel and 3 mL of AF-70 to a multimixer. Then, the diesel andemulsifying agent were mixed at a shear rate from 400 to 1000 rpm for 1minute. Next, 210 mL of a 20 wt. % hydrochloric acid solution were addeddropwise over 45 minutes while the diesel, emulsifying agent, and acidmixture was mixed at a shear rate of from 1000 to 2000 rpm forming aninvert emulsion.

Next, 90 mL of water and 3 mL LoSurf-300, a commercially availableemulsifying agent from the Halliburton Company with an HLB less than orequal to 7, are added to a multimixer and mixed at a shear rate from 400rpm to 1000 rpm for 1 minute. Next, 210 mL of the previously preparedinvert emulsion are added dropwise over 45 minutes while the mixture wasmixed at a shear rate of from 1000 to 2000 rpm forming a doubleemulsified acid. The produced double emulsified acid was imaged under amicroscope and photographed. As can be seen in FIG. 1, a double emulsionis formed with an aqueous acid phase being entirely surrounded by an oilphase; and the oil phase is entirely surrounded by a continuous aqueousphase. The double emulsion micelles depicted in FIG. 1 have a diameterin the range of from 1 to 20 millimeters.

Viscosity Measurement

The viscosities of Comparative Example A and Example 1 were measured at25° C. using a FANN-35 viscometer at a shear rate of 100 s⁻¹.Comparative Example A had a viscosity of 75 centipoise (cP) and Example1 had a viscosity of 56 cP. The less viscous double emulsified acid canbe pumped at faster rates than the more viscous conventional emulsifiedacid. Also related to its rheological properties, the less viscousdouble emulsified acid is more efficient to pump into HTHP environmentscompared to more viscous conventional emulsified.

Carbonate Reaction Rate

The reaction rates of Comparative Example A with calcium carbonate andExample 1 with calcium carbonate were measured and compared at 25° C. Inan container, 10 grams (g) of CaCO₃ powder were added to 100 mL ofComparative Example A and allowed to react for four different timeintervals: 30 seconds, 1 minute, 2 minutes, and 5 minutes. At the end ofeach time interval, the solution was filtered and the remainingundissolved mass was measured and subtracted from the initial CaCO₃ massto determine the mass of CaCO₃ reacted.

Similarly, 10 g of CaCO₃ powder were added to 100 mL of Example 1 andallowed to react for four different time intervals: 30 seconds, 1minute, 2 minutes, and 5 minutes. As the CaCO₃ reacted with the doubleemulsified acid, CO₂ was evolved. At the end of each time interval, thesolution was filtered and the remaining undissolved mass was measuredand subtracted from the initial CaCO₃ mass to determine the mass ofCaCO₃ reacted. The masses of reacted CaCO₃ in Comparative Example A andExample 1 at time intervals of 30 seconds, 1 minute, 2 minutes, and 5minutes, are shown in Table 1.

TABLE 1 Comparative Example A Example 1 Elapsed Time CaCO₃ Mass ReactedCaCO₃ Mass Reacted 0.5 minutes 0.5 g 0.1 g 1 minute 1.4 g 0.4 g   2minutes 2.5 g 1.3 g   5 minutes 7.5 g 2.65 g 

As can be seen from Table 1, the double emulsified acid dissolved CaCO₃at a rate slower than the emulsified acid. This is because the doubleemulsified acid is reacting with the carbonate slower than theconventional emulsified acid.

Further, it will be apparent that modifications and variations arepossible without departing from the scope of the present disclosure,including, but not limited to, embodiments defined in the appendedclaims. More specifically, although some aspects of the presentdisclosure are identified as particularly advantageous, it iscontemplated that the present disclosure is not necessarily limited tothese aspects.

Unless otherwise defined, all technical and scientific terms used inthis disclosure have the same meaning as commonly understood by one ofordinary skill in the art. The terminology used in the description isfor describing particular embodiments only and is not intended to belimiting. As used in the specification and appended claims, the singularforms “a,” “an,” and “the” are intended to include the plural forms aswell, unless the context clearly indicates otherwise.

It should be understood that any two quantitative values assigned to aproperty may constitute a range of that property, and all combinationsof ranges formed from all stated quantitative values of a given propertyare contemplated in this disclosure. It should be further understoodthat any and all ranges provided are inclusive of their respectiveendpoints.

It will be apparent to those skilled in the art that variousmodifications and variations may be made to the embodiments describedwithin without departing from the spirit and scope of the claimedsubject matter. Thus, it is intended that the specification cover themodifications and variations of the various embodiments described withinprovided such modification and variations come within the scope of theappended claims and their equivalents.

What is claimed is:
 1. A double emulsified acid comprising: from 5 wt. % to 55 wt. % of a continuous aqueous phase, based on the total weight of the double emulsified acid; a non-continuous invert emulsion phase, the non-continuous invert emulsion phase comprising: from 10 wt. % to 30 wt. % of an exterior oil phase, based on the total weight of the double emulsified acid; from 5 wt. % to 65 wt. % of an internal acid phase, based on the total weight of the double emulsified acid, the internal acid phase comprising: from 1 wt. % to 99 wt. % of at least one acid, based on the total weight of the internal acid phase; and from 1 wt. % to 99 wt. % water, based on the total weight of the internal acid phase; and from 10 wt. % to 20 wt. % of a first emulsifying agent, based on the total weight of the double emulsified acid, the first emulsifying agent having a hydrophilic-lipophilic balance (HLB) greater than 7; and from 10 wt. % to 20 wt. % of a second emulsifying agent based on the total weight of the double emulsified acid, the second emulsifying agent having an HLB less than or equal to
 7. 2. The double emulsified acid of claim 1, where at least one of the emulsifying agents comprises an ethoxylated alcohol.
 3. The double emulsified acid of claim 1, where at least one of the first and second emulsifying agents comprises an alkylphenol.
 4. The double emulsified acid of claim 1, where at least one acid comprises an acid having a pK_(a) less than or equal to
 5. 5. The double emulsified acid of claim 1, where at least one acid comprises acetic acid, chloric acid, formic acid, hydrochloric acid, hydrobromic acid, hydrofluoric acid, hydroiodic acid, nitric acid, oxalic acid, perchloric acid, sulfuric acid, sulfurous acid, or combinations thereof.
 6. The double emulsified acid of claim 1, where the internal acid phase comprises from 25 wt. % to 75 wt. % of at least one acid, based on the total weight of the internal acid phase.
 7. The double emulsified acid of claim 1, where the exterior oil phase comprises mineral oil, safra oil, diesel, C₁₂C₂₀ hydrocarbons, or combinations thereof.
 8. The double emulsified acid of claim 1, wherein the non-continuous invert emulsion phase is entirely surrounded by the continuous aqueous phase. 